Properties of Nd-Fe-B Magnetic Powder Prepared by Reduction-Diffusion Process Using a Spray-Dried Precursor

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Effect of washing step in R-D process on the structure and magnetic properties of Nd-Fe-B
powder fabricated by the process using a spray-dried precursor was investigated. Although the
powder washed in water contained much more residual CaO than that washed in dilute acetic acid,
magnetic properties of the former were much better than those of the latter due to less removal of
Nd-rich phase. In the powder washed in water Nd2Fe14B particles were enclosed with thin layer of
Nd-rich phase, and each particle consisted of one or more subgrains of the size 50 ~ 100 nm or more.
Milling the powder before water washing was effective to remove more CaO, thereby increased the
remanence of the powder.

Abstract: In the present work, we investigated the hard magnetic properties of Nd-Fe-B magnets prepared by the decrepitation technique. The microstructure of (Nd0.935Dy0.065)14.5Fe79.4B6.1 HD - magnet was observed and the average grain size is about 6-15 µm. The HD alignment degree f is about 93%. Most rich-Nd phase can be found at grain boundaries of main phase, but a few rich – Nd phase occurs within main phase grains. The magnetizing field dependence of the hardmagnetic properties for the (Nd0.935Dy0.065)14.5Fe79.4B6.1 were investigated. The value of magnetizing field for the saturation coercivity is smaller than the value of the saturation coercivity, showing that the coercivity for the (Nd0.935Dy0.065)14.5Fe79.4B6.1 HD-magnet is controlled by the nucleation
mechanism. Meanwhile, the temperature dependence of coercivity for HD magnet Nd14.5Fe79.4B6.1 was analyzed.

Abstract: Nanocrystalline YCo5 powders with high coercivity were prepared by mechanical milling and subsequent heat treatment at 820 °C for different annealing times, ta = 2.5, 3.0, 3.5 and 4.5 min, obtaining average crystallite sizes of  17, 19, 32 and 39 nm., respectively. The coercivity values were determined from the hysteresis loops measured at maxima fields of Hm = 5 and 20 T. The highest coercivity was obtained for the sample exhibiting  19 nm, where at room temperature and Hm = 5 T, the coercivity value is of 9.0 kOe. At 77 K and Hm = 5 T, the coercivity increase to 11.8 kOe and for Hm = 20 T, a higher value such as 13.1 kOe was found. The Ms/Mr ratio is enhanced to 0.62 indicating the occurrence of exchange interaction among nanocrystalline magnets.

Abstract: The multiferroic Aurivillius phases in the Bi-Fe-Ti-O system are built from alternate (Bi2O2)2+ and (Bin-1XnO3n+1)2 layers, where X = Fe3+, Ti4+ and “n” refers to the number of perovskite-like layers between Bi2O2 layers. Detailed magnetic studies should be done to understand electromagnetic interactions and multiferroic coupling effects. In the present paper, a powder composed of the Aurivillius phase with n = 5, Bi6Fe2Ti3O18, was successfully prepared by the hydrothermal method. The powder was sintered, obtaining dense polycrystalline materials. It was stated that both powder and sintered bodies were paramagnets with a possible antiferromagnetic ordering or a spin-glass state at the liquid helium temperatures.

Abstract: In the present work a study of the influence of structural state on magnetic hysteresis properties of (Nd,Ho)2(Fe0.8Co0.2)14B compound was carried out. Starting alloy was prepared by induction melting in an Ar atmosphere. Nd-Ho-Fe-Co-B alloys with a nanograin structure were obtained by severe plastic deformation (SPD). Electron microscopy and X-ray analysis were used for the structural investigation. The magnetization measurements were performed using a SQUID magnetometer. It is shown that the relatively high values of coercive force are observed in case of achieved nanograin structure in (Nd,Ho)2(Fe0.8Co0.2)14B. The effect of structural state on hysteresis properties of Y2(Fe0.8Co0.2)14B is also investigated.

Abstract: Influence of nanoflakes and melt-spun ribbons on the nitrogenation temperature f Sm2Fe17 was investigated. The microstructure, phase structure, properties of the specimens before and after nitrogenation have been discussed. It is interesting found that hard magnetic Sm2Fe17N3 flakes have been synthesized by surfactant-assisted high energy ball milling and nitriding Sm2Fe17 in 3 psi of N2 gas at lower temperature 300 °C and 350 °C for 3 h. The synthesized Sm2Fe17N3 flakes still retained nanostructure and obtained a coercivity of 3.56 kOe. For the melt-spun ribbons, it needs a higher nitrogenation temperature and longer time to obtain the same coercivity values as the nanoflakes.